1. Field of the Invention
This invention relates to a process for producing a crosslinked polyethylene insulated cable, particularly a high voltage cable having an easily removable outer semiconductive layer.
2. Description of the Prior Art
A high voltage cable comprises an electrical conductor and formed thereon an internal semiconductive layer, an electrically insulating layer and an outer semiconductive layer. The last layer serves to alleviate or shield the surroundings from an electric field generated by the electrical conductor.
According to conventional techniques, this outer semiconductive layer is formed by winding an electrically conductive tape around the cable or by extrusion-coating thereon a composition obtained by mixing polyethylene, an ethylene/ethyl acrylate copolymer or an ethylene/vinyl acetate copolymer with electrically conductive carbon black and other additives such as talc, clay, calcium carbonate, magnesium oxide, zinc oxide, magnesium zinc salts, anti-oxidants or crosslinking agents. The use of tapes has the defect that the poor adhesion between the tapes and an insulator adversely affects the electrical properties of the cable. On the other hand, in processing the ends of a cable to form a junction, for instance, it is necessary to remove the outer semiconductive layer to a predetermined length and an extrusion-coated layer of the semiconductive composition cannot be as easily removed as the tape. The extrusion-coated outer semiconductive layer must therefore be shaved off. However, much time and care are required to remove the outer semiconductive layer without damaging the surface of the insulator.
Outer semiconductive layers which adhere well to the insulator but can be easily removed at the time of working the cable ends have been developed (for example, as disclosed in U.S. Pat. Nos. 3,719,769 and 3,684,821). Such outer semiconductive layers are made by kneading conductive carbon black with an ethylene/vinyl acetate copolymer (EVA for short), a copolymer of EVA and vinyl chloride (EVA-PVC for short), or a mixture of EVA and EVA-PVC, and can be easily peeled off upon working the cable ends without damaging the surface of the insulators. Moreover, these semiconductive layers do not separate from the insulators when the cables are used. These conductive layers have sufficient peelability and processability for practical purposes, however, even with these outer semiconductive layers sometimes the semiconductive layer cannot be completely removed and areas remain on the insulator after removal. In such a case, the remaining conductive layer must be removed by shaving or by wiping it off with a solvent. Furthermore, peroxide is added to these semiconductive layer compositions to effect crosslinking thereof such that the semiconductive layer has sufficient strength as an outer semiconductive layer (ordinarily about 0.5 to about 5 phr). Under some extrusion-processing conditions, small protrusions, termed "scorch" form on the surface of the outer semiconductive layer or between the outer semiconductive layer and the insulator at the time of producing the cables. When extruding compositions containing carbon, the temperature of the material increases due to heat generation by shearing and when a crosslinking agent is present in the composition crosslinking is often initiated by the heat thus generated which causes scorch and because of this it is very difficult to select conditions for extrusion when a composition contains carbon and a crosslinking agent.
On the other hand, in the conventional techniques in which polyethylene used for producing crosslinked polyethylene insulated cables, the cables are heated in most cases to about 200.degree. C. to effect crosslinking. It is apparent that higher crosslinking temperature is desirable since it leads to a higher crosslinking speed and this in turn yields greater economical advantages. However, it has been difficult to further increase crosslinking speed when using conventional resin compositions since the resulting outer semiconductive layer cannot be peeled off with ease when the resin composition is heated to 230.degree. C. or more although it is still peelable when the resin composition is heated at 200.degree. C. The reason this phenomenon is observed is not completely clear at present but it is believed that it is ascribable to a relative decrease in the tensile strength of the outer semiconductive layer due to thermal deterioration of the resin material used therein with respect to peel strength of the outer semiconductive layer.
Thus, there has been increasing demand for crosslinked polyethylene insulated cables the outer semiconductor of which can be peeled off by hand without using special tools.